The present invention relates to imaging techniques and apparatus for performing such techniques, and more specifically to techniques and apparatuses for assisting patient positioning while performing such imaging techniques are performed.
In magnetic resonance imaging (“MRI”), a strong, uniform magnetic field is applied to the region of the patient to be imaged. Radio frequency (“RF”) energy is applied to this region of the patient by a transmitter and antenna. The RF energy excites atomic nuclei within the patient's tissues. The excited nuclei spin at a rate dependent upon the magnetic field. As they spin, they emit faint RF signals, referred to herein as magnetic resonance signals. By applying small magnetic field gradients so that the magnitude of the magnetic field varies with location within the patient's body, the magnetic resonance phenomenon can be limited to only a particular region or “slice” of the patient's body, so that all of the magnetic resonance signals come from that slice. Moreover, by applying additional magnetic field gradients, the frequency and phase of the magnetic resonance signals from different locations within the slice can be made to vary in a predictable manner depending upon the position within the slice. Stated another way, the magnetic resonance signals are “spatially encoded,” so that it is possible to distinguish between signals from different parts of a slice.
If this process is repeated numerous times to elicit signals using different gradients, it is possible to derive a set of information which indicates one or more characteristics of magnetic resonance signals from particular locations within the patient's body. Such a set of information is referred herein to as an image data set. Because the characteristics of the magnetic resonance signals vary with the concentration of different chemical substances and other chemical characteristics of the tissues, different tissues provide different magnetic resonance signal characteristics. When a magnetic resonance signal image data set is displayed in a visual format, such as on a computer screen or printed image, the information forms a picture of the structures within the patient's body, with different tissues having different intensities or colors.
Typically, a magnetic resonance image data set is stored as a set of individual data elements. The data in each element represents one or more characteristics of magnetic resonance signals from a small volume element or “voxel.” For example, the map can be stored as a three-dimensional array of data elements, the dimensions of the array corresponding to three-dimensional space. Data elements corresponding to a given plane in three-dimensional space can be selected for display in a two-dimensional picture such as a screen display or printed image. Each small area element on the surface of the picture, commonly referred to as a “pixel,” is assigned an intensity or color value based on the numerical values of the data element for the corresponding voxel.
MRI has been widely adopted in the medical arts. Because MRI does not use X-rays or other ionizing radiation, it offers safety advantages over techniques such as conventional X-ray imaging, fluoroscopy and CAT imaging. Moreover, MRI allows visualization of tissues which are difficult or impossible to depict using other techniques. Magnetic resonance imaging can show abnormal tissues in contrast to surrounding normal tissues. MRI is also particularly useful in imaging the spine. MRI can depict the vertebrae in conjunction with related tissues such as the lamina or “discs,” as well as nerves, muscles and other neighboring tissues.
However, magnetic resonance imaging procedures have suffered from significant limitations. Conventional MRI equipment requires the patient to lie in a supine position on a horizontal bed which fits with the patient-receiving space of the static field magnet. Some medical conditions have effects which change with posture. For example, a spinal disc may impinge on a nerve or other surrounding structure only when the patient is in an upright posture so that the disk is compressed by the patient's weight. Various proposals have been advanced to allow MRI procedures to be performed on patients in a posture other than the conventional supplying of posture. For example, Japanese published Patent Application No. 1-242056 published Sep. 27, 1989 depicts a magnetic resonance imaging unit with a tilting bed for supporting the patient in a supine position or in a standing position. Yoshida, U.S. Pat. No. 5,008,624 depicts a magnetic resonance imaging instrument with movable static field magnet in conjunction with a patient carrier which supports the patient in “various postures.” Palkovich et al., U.S. Pat. No. 5,779,637 discloses a system in which the patient lies supine within the static field magnet during one imaging procedure. The entire system, including the static field magnet and the patient can be pivoted so as to swing the magnet, the patient bed and the patient as a unit to a different position in which the patient bed extends vertically and the patient in an upright posture. A further image is taken in this position. None of these systems have been widely adopted.
Copending, commonly assigned U.S. patent application Ser. No. 09/718,946, filed Nov. 22, 2000 (“the '946 application”), the disclosure of which is hereby incorporated by reference herein and copending commonly assigned U.S. patent application Ser. No. 09/789,460 (“the '460 application”) the disclosure of which is also incorporated by reference herein describe additional MRI magnet structures and patient handling devices as well as additional imaging methods. As disclosed for example in certain embodiments of the '946 application, a patient support such as a bed which can both tilt and elevate can be used in conjunction with a static field magnet to allow imaging of a patient in various orientations and to position various portions of the patient's anatomy in the appropriate location relative to the magnet for imaging. Discussion of the '946 and '460 applications in this background section of the present application should not be taken as an admission that the same constitute legally available prior art with respect to the present invention.
The patient support system including the bed and the tilt and elevate mechanisms disclosed in the '946 application can place a patient in positions from which a patient may have a fear of slipping or falling, regardless of whether that fear is justified. For example, a standing patient's longitudinal axis is substantially vertical, the patient may have a fear that he or she will fall away from the bed. The patient may be stabilized, supported or restrained to prevent the patient from falling off the bed, or to give the patient further reassurance. While it is possible to restrain the patient by strapping the patient to the bed, this may be inconvenient and may be physically uncomfortable or disturbing to the patient. For example, one desirable implementation of the system disclosed in the '946 application is a “walk-in” imaging procedure in which a patient can walk onto a footrest associated with the bed, step onto the footrest and lean against the bed as the bed moves and elevates. A significant advantage of this procedure is that it is convenient and psychologically non-threatening to the patient. If the patient has to be strapped to the bed, this advantage is reduced.